US President Donald Trump on Wednesday delayed a second summit with Russia's Vladimir Putin, while his top diplomat insisted there was no let up against Moscow following the two leaders' controversial meeting in Helsinki. 

Facing mounting calls to release the details of Trump's closed-door talks with Putin, the US administration stepped up its damage control operation, with Secretary of State Mike Pompeo going before Congress to defend his boss.

The US president himself, under fire for plans to invite Putin to the White House in the fall, opted to delay until 2019 -- pushing the talks back until Robert Mueller has completed his probe into Moscow's election interference, according to National Security Advisor John Bolton.

And Pompeo went on the offensive to stress steps Trump has taken to show resolve against the Kremlin, stepping into a white-hot spotlight during a three-hour grilling by the Senate Foreign Relations Committee.

The top diplomat said he would back bipartisan efforts in Congress to slap new sanctions on Russia in response to their meddling in the 2016 US election, and as a deterrent against meddling this year or in 2020.